The isoparaffin/olefin alkylation process is widely used to manufacture a high octane quality blend component for aviation and motor gasoline which is also valued for its relatively low vapor pressure, low sensitivity and, because of its freedom from sulfur and aromatic components, its environmental acceptability. The process typically reacts a C2 to C5 olefin with a light (C4 to C5) isoparaffin, typically isobutane, in the presence of an acidic catalyst to produce the alkylate product.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal but in spite of efforts to develop an inherently safe alkylation process, both processes have achieved widespread utilization with the HF process being noted for producing a higher quality product with more favorable unit economics. Although hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive, extensive experience in its use in the refinery have shown that it can be handled safely, provided the hazards are recognized and precautions taken. The HF alkylation process is described in general terms in Modern Petroleum Technology, Hobson et al (Ed), Applied Science Publishers Ltd. 1973, ISBN 085334 487 6. A survey of HE alkylation may be found in Handbook of Petroleum Refining Processes, Meyers, R. A. (Ed.). McGraw-Hill Professional Publishing, 2nd edition (Aug. 1, 1996), ISBN: 0070417962.
Monitoring acid strength is necessary to optimize alkylation product, reduce operating expenses, and prevent unit upsets. Acid strength effects the yield and octane number (quality) of the alkylate product. Low acid strength levels result in so-called acid runaway, where olefin polymerization is favored over the desired alkylation reaction, which further lowers the acid strength with the addition of the by-product polymer (acid soluble oil, ASO) to the mixture and leads to a positive feedback cycle rapidly consuming the acid catalyst and producing polymer sludge and reaction intermediates. A common result of acid runaway is unit shutdown.
A similar problem of making on-line measurement of acid strength is found in sulfuric acid alkylation units, where sulfuric acid strength measurements are also required for optimal operation. With the high relative density of sulfuric acid (1.84) relative to water and ASO (1 and ˜0.7-0.9, respectively), which are the two other primary components in the system, it is common to use density to quantify sulfuric acid strength. U.S. Pat. No. 5,707,923 (Hutchens) discloses the use of density measurements on the reaction mixture for the determination of operating limits including acid strength. Because the density differences in HF units are not as great as with sulfuric acid, density measurement is not commonly used to measure HF acid strength; in addition, material incompatibilities between many metals commonly found in densitometers and HF preclude its use in any extensive practice.
U.S. Pat. No. 7,972,863 (Trygstad) discloses a method for determining the concentration of components in the liquid mixture flowing through the alkylation process by measuring certain properties of the mixture which are independent of the concentration of the water in the mixture; the temperature of the mixture is also measured to enable a temperature compensated concentration to be determined.
Another technology for on-line acid strength measurement is marketed by K-Patents Oy (Finland) and is based upon refractive index measurements. The company has sold the product for sulfuric acid strength measurements but so far, refractive index measurements have not been successfully implemented in a commercial setting for measuring HF acid strength even though the refractive index of HF (1.15) is much lower than water (1.33) or ASO (1.4), which in principle could allow changes in acid strength to be inferred from changes in refractive index. Material incompatibilities with HF have, however, been a negative factor to utilizing the technique in HF alkylation units.
Currently, acid strength measurements in HF alkylation units are most commonly made by manual sampling and laboratory analysis. The analysis is time consuming and typically performed at most once per day; more commonly it is performed only hi-weekly or even weekly. At such low analysis frequencies, it is difficult to monitor rapidly changing process conditions. Manual analysis presents a legitimate safety risk. Hydrofluoric acid is highly toxic and extremely volatile. Contact with the skin causes severe burns and is readily absorbed into the body, where it reacts with calcium causing bone damage and possibly cardiac arrest. Inhaling gaseous hydrofluoric acid can cause irreversible lung damage at concentrations over 10 ppm.
The only commercially-implemented automated analysis technique used to quantify HF acid strength uses Infrared (IR) Spectroscopy. IR Spectroscopy is a relatively complicated technique that is difficult and costly to implement in an alkylation unit. The IR system requires an automated sample conditioning system, which has many moving parts and possible leak sources. To mitigate the possibility of a leak involving HF, the IR system has integrated HF detectors that are tied into an automated shutdown system that closes sample supply and evacuates the system with nitrogen. In addition IR spectroscopy is maintenance intensive and requires an air-conditioned, vibration-free environment. Typical cost of an IR system including installation is more than $2 M USD.